The instant invention relates to transparent iron oxide pigments. More specifically, the instant invention relates to a novel process for conversion of yellow transparent iron oxide to red transparent iron oxide.
Iron oxide pigments are found in nature as mineral deposits. There are three types of iron oxide pigments which are found in nature as mineral deposits. These are limonite, hematite, and magnetite. Limonite is yellow and has a chemical formula of Fe.sub.2 O.sub.3.xH.sub.2 O where x is an integer. As can be seen from its formula, it is a hydrated iron(III)oxide. Hematite is red and has a chemical formula of Fe.sub.2 O.sub.3. It is an anhydrous iron(III)oxide. Magnetite is black and has a chemical formula of Fe.sub.3 O.sub.4. It is considered to be a spinal containing iron(II) and iron(III)ions.
In addition to the natural mineral deposits, iron oxide pigments can be obtained synthetically. The prior art preparation of iron oxide pigments is well known. Reference is made to various topics concerning iron oxide such as "Pigments (Inorganic)," pages 816-818, vol. 17, Encyclopedia of Chemical Technology, by Kirk-Othmer, 3rd edition, published by John Wiley & Sons.
Iron oxide pigments which have an average particle size less than 0.1 micron are considered to be transparent because they can transmit visible light. Stated another way, "transparent pigment" is pigment having a majority of the particles smaller than the wave length of light. Iron oxide pigments which have an average particle size greater than 0.1 microns and which cannot transmit light are considered to be opaque.
Synthetic methods for preparing iron oxide pigments generally involve the alkali precipitation of iron(II) compounds from a soluble iron(II)salt and the oxidation of the precipitated iron(II) compound to an iron(III)oxide pigment slurry. The pigment produced by such methods corresponds to the yellow hydrated iron(III)oxide previously described. The yellow pigment is recovered from the slurry by filtration, washing, and drying. Red transparent iron oxide can be prepared by calcining the yellow pigment at high temperatures.
As can be seen from the above the prior art method for preparing a red transparent iron oxide pigment from a yellow transparent iron oxide pigment filter cake requires two steps, i.e., drying and calcining. The final red oxide pigment product is then prepared by grinding the calcined pigment. Accordingly it is a purpose of the instant invention to accomplish both drying and calcining in a single step.